The Minnesota Population Center is near completion of a one-percent public use microdata sample of the 1930 census (HD41575). This proposal seeks continuation funding to expand the 1930 sample from 1,222,727 cases to approximately 6,200,361 cases, representing five percent of the population. Because of work carried out for the one-percent microdata sample, an additional four percent of the population can be added at minimal expense. - Similar five-percent census .microdata samples are available for the period since 1980. Scholars are increasingly capitalizing on the power of these large samples to study everything from scientists in two- career marriages to multi-race American Indians to father-only families. Indeed, the five-percent samples from the period i960 through 2000 have become the single most widely used data source in Demography, the leading American journal of population research. Historical research based on census microdata is also growing rapidly. The Integrated Public Use Microdata Series (IPUMS), a series of census microdata spanning the period from 1850 to 2000, has generated almost a thousand peer- reviewed publications and Ph.D. dissertations during the past decade. This work has opened important new avenues of research that have expanded our understanding of migration, nuptiality, fertility, the family, and labor markets. The intensive use of the high-density samples of the recent censuses has shown the great value of large samples, and the track record of the IPUMS has shown the importance of historical microdata. The Minnesota Population Center is working on new high-density samples for the 1880, 1900, and 1960 census years. An expanded sample for 1930 will fill a critical gap between the 1900 and 1960 censuses, allowing in-depth analyses at thirty-year intervals. These data will open a broad range of new research topics and methodological approaches to the study of long-run demographic change, varying from the study of the oldest-old to multi-level analysis. This expanded database is directly relevant to the central mission of the NIH as the steward of medical and behavioral research for the nation: the new data will advance fundamental knowledge about the nature and behavior of human population dynamics. This basic infrastructure will advance health-related research on population growth and movement, fertility, mortality, and nuptiality, as well as the economic and social correlates of demographic behavior.